if the rings were gone there would be loads of smoke out the exhaust getting worse as the oil heates up,

if it is stugling to idle when stood still but running on all 4 cylinders i'd say it was a fault with the idle control valve as these have been shit on fords since they first used them in late 80's early 90's.

btw any fluid on the spark plugs will look black as the cylinders are full off soot and turn everyhing black just like youe exhaust pipe etc.